1. Field of the Invention
This invention relates to electrolyte membranes suitable for use in proton exchange membrane fuel cells. More particularly, this invention relates to low cost polystyrene membranes having high stability for use in proton exchange membrane fuel cells.
2. Description of Related Art
A fuel cell is an electrochemical device in which the chemical energy of a reaction between a fuel and an oxidant is converted directly into electricity. The basic fuel cell unit comprises an electrolyte layer in contact with a porous anode and cathode on either side. In a typical fuel cell, a gaseous or liquid fuel is continuously fed to the anode electrode, sometimes referred to as the fuel electrode, and an oxidant, such as oxygen from air, is continuously fed to the cathode electrode, sometimes referred to as the air electrode, and electrochemical reactions occur at the electrodes to produce an electric current. Due to the limited electricity generating capacity of individual fuel cell units, a plurality of fuel cell units are typically stacked one on top of another with a bipolar separator plate separating the fuel cell units between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit.
There are a number of different fuel cell types which are classified based upon a variety of categories including the combination of type of fuel and oxidant, whether the fuel is processed external to or inside the fuel cell, the type of electrolyte, e.g. solid oxides, phosphoric acid, molten carbonate and proton exchange membranes, the temperature of operation and whether the reactants are provided to the fuel cell by internal or external manifolds.
This invention relates to proton exchange membrane fuel cells. In a proton exchange membrane fuel cell, also sometimes referred to as a polymer electrolyte membrane fuel cell, the electrolyte is a proton conducting membrane sandwiched between two porous electrodes. The back of the electrodes is made hydrophobic by coating with an appropriate compound, such as TEFLON®. Proton conducting membranes conventionally used in proton exchange membrane fuel cells are made of a perfluorinated sulfonic acid polymer, an example of which is sold under the brand name NAFION® by DuPont. NAFION membranes, which are fully fluorinated polymers, have exceptionally high chemical and thermal stability and are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, Cl2, H2 and O2 at temperatures up to about 100° C. NAFION consists of a fluoropolymer backbone upon which sulfonic acid groups are chemically bonded. However, although an exceptional performer, NAFION is an expensive material and makes proton exchange membrane fuel cells economically unattractive in most applications. Much of the cost of NAFION is due to two factors: the use of fluorine and the very severe reaction conditions needed to prepare the polymer.
Accordingly, the challenge is to find lower cost membranes having the desired properties for use as a proton conductor in proton exchange membrane fuel cells. Some of these properties include mechanically stable and durable film behavior in the cell-operating environment with long lifetimes, hydrophilicity for high conductivity and water insolubility.
Low cost membranes in the form of sulfonated polystyrene membranes have been applied to proton exchange membrane fuel cells. U.S. Pat. No. 5,468,574 and U.S. Pat. No. 5,679,482, both to Ehrenberg et al., teach an ion-conducting membrane for a fuel cell comprising a plurality of acid-stable polymer molecules, each having at least one flexible connecting component. The membrane, which is composed of hydrogenated and sulfonated styrene copolymers, is obtained by sulfonating a styrene-(ethylene-butylene)-styrene (SEBS) copolymer to the extent of at least 25 mol %. Less than a stoichiometric amount of sulfonation agent is employed; preferably the amount of sulfonation agent employed is enough to theoretically react with at least about 15% of the available styrene molecular units and preferably in the range of about 40% to about 60% of the available styrene molecular units. Higher sulfonation levels are indicated to decrease the strength of the membranes when they become hydrated. Similarly, U.S. Pat. No. 6,110,616 to Sheikh-Ali et al. teaches an ion-conducting membrane for a fuel cell, which membrane is composed of a hydrogenated and sulfonated statistical copolymer of styrene and butadiene obtained by hydrogenating a copolymer of styrene and butadiene to obtain less than 5% residual unsaturation, then sulfonating the polymer with an acetyl sulfate sulfonation agent to a level of at least 30 mol % sulfonate. The challenge is to improve the stability of these membranes.